Three approaches were taken to address the mechanism of cellular transformation induced by nonreceptor protein-tyrosine kinases. one involves overexpression of fgr genes specifying normal or aberrant kinases in NIH/3T3 cells. The findings document a rare malignant transformation by high levels of p55(c-fgr). Furthermore, it was shown that by mutation of sequences encoding the carboxyl terminus of p55(c-fgr), the c-fgr gene is converted into a potent, dominant acting oncogene. A search for identify of substrates for these activated tyrosine kinases shows substrate criteria to include physical association with the enzyme in vivo, as well as tyrosine phosphorylation by the enzyme both in vivo and in vitro. Using these criteria molecules of 135 kd and 70 kd that preferentially interact with and are tyrosine phosphorylated by transforming as compared to normal versions of src, fyn, and fgr kinases were identified. The third approach involves searching for evidence of activated tyrosine kinases in naturally occurring human neoplasia, especially squamous cell carcinomas of the head and neck. Sensitive assays are being developed for activation, including PI-3 kinase association and tyrosine phosphorylation of certain substrates in vitro and in vivo in order to enhance the probability of identifying such lesions.